1. Field of the Invention
The present invention relates to a circuit for controlling RF PAs (Radio Frequency Power Amplifiers), and more specifically, to an RF PA controller circuit that adjusts the supply voltage of RF PAs.
2. Description of the Related Art
RF (Radio Frequency) transmitters and RF power amplifiers are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF power amplifiers are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life of these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA systems, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time.
FIG. 1 illustrates a conventional RF transmitter circuit, including a transmitter integrated circuit (TXIC) 102 and an external power amplifier (PA) 104. In some cases, there may be a filter between the TXIC 102 and the PA 104. For example, the RF transmitter circuit may be included in a cellular telephone device using one or more cellular telephone standards (modulation techniques) such as UMTS (Universal Mobile Telephony System) or CDMA (Code Division Multiple Access), although the RF transmitter circuit may be included in any other type of RF electronic devices. For purposes of illustration only, the RF transmitter circuit will be described herein as a part of a cellular telephone device. The TXIC 102 generates the RF signal 106 to be amplified by the PA 104 and transmitted 110 remotely by an antenna (not shown). For example, the RF signal 106 may be an RF signal modulated by the TXIC 102 according to the UMTS or CDMA standard.
The RF power amplifier 104 in general includes an output transistor (not shown) as its last amplification stage. When an RF modulated signal 106 is amplified by the PA 104, the output transistor tends to distort the RF modulated signal 106, resulting in a wider spectral occupancy at the output signal 110 than at the input signal 106. Since the RF spectrum is shared amongst users of the cellular telephone, a wide spectral occupancy is undesirable. Therefore, cellular telephone standards typically regulate the amount of acceptable distortion, thereby requiring that the output transistor fulfill high linearity requirements. In this regard, when the RF input signal 106 is amplitude-modulated, the output transistor of the PA 104 needs to be biased in such a way that it remains linear at the peak power transmitted. This typically results in power being wasted during the off-peak of the amplitude of the RF input signal 106, as the biasing remains fixed for the acceptable distortion at the peak power level.
Certain RF modulation techniques have evolved to require even more spectral efficiency, and thereby forcing the PA 104 to sacrifice more power efficiency. For instance, while the efficiency at peak power of an output transistor of the PA 104 can be above 60%, when a modulation format such as WCDMA is used, with certain types of coding, the efficiency of the PA 104 falls to below 30%. This change in performance is due to the fact that the RF transistor(s) in the PA 104 is maintained at an almost fixed bias during the off-peak of the amplitude of the RF input signal 106.
Certain conventional techniques exist to provide efficiency gains in the PA 104. One conventional technique improves the efficiency in the PA 104 by lowering the supply voltage 108 to the PA 104 provided by a power supply such as the switched mode power supply (SMPS) 112. By using a lower supply voltage 108, the PA 104 operates with increased power efficiency because it operates closer to the saturation point. However, the supply voltage 108 cannot be reduced too low, because this would cause the PA 104 to operate with insufficient voltage headroom, resulting in unacceptable distortion. As described previously, the distortion may cause energy from the transmitted signal to spill over to adjacent channels, increasing spectral occupancy and causing interference to radios operating in those neighboring channels. Thus, an optimal supply voltage should be chosen for the PA which balances acceptable distortion with good efficiency.
One conventional method uses a fixed output voltage step-down regulator such as switched mode power supply (SMPS) 112 to lower the supply voltage 108 to the PA 104. However, choosing a fixed power supply voltage is not sufficient in many applications. For example, in most cellular systems, the PA output power changes frequently because the base station commands the cellular handset to adjust its transmitted power to improve network performance, or because the handset changes its transmitted information rate. When the PA output power changes, the optimum supply voltage for the PA (as described above) changes.
Therefore, in some systems, the expected power of the RF output signal 110 is first determined, and then the power supply voltage 108 is adjusted in accordance with the expected power. By adaptively adjusting the supply voltage 108, the efficiency of the PA 104 is increased across various PA output power levels. Conventional methods estimate the expected power of the RF output signal 110 in an “open loop” manner, in which the power of the RF output signal 110 is estimated from the power of the delivered RF input signal 106. However, an estimate of the power of the RF output signal 110 may still not be sufficient for properly adjusting the supply voltage 108. For example, the peak-to-average ratio (PAR) needs to be known in order to estimate the optimum supply voltage for the PA. The PAR refers to the difference of the mean amplitude and the peak amplitude in the modulated RF output signal 110. With a higher PAR, a higher supply voltage is needed to accommodate the peak voltage swings of the RF output signal 110. Many modern cellular systems change the PAR of the modulation in real time, requiring the supply voltage to be adjusted accordingly. Therefore, the conventional method of adjusting the supply voltage 108 of PA 104 based on an estimate of the PA output power is unsuitable in these cellular systems.
Further, the load presented to the PA 104 poses another significant problem. The PA 104 normally drives circuitry usually consisting of a filter and an antenna. Such circuitry typically has nominal impedance around the range of 50 ohms. However, the impedance of the circuitry can sometimes change radically from the nominal. For example, if the antenna is touched or the cellular device is laid down on a metal surface, the impedance of the antenna changes, reflecting impedance changes back to the PA 104. The changes in the impedance of the circuitry coupled to the PA 104 may require changes in the supply voltage to the PA 104 to prevent distortion of the RF output signal 110 fed to this circuitry. The conventional methods described above, however, do not adjust the supply voltage in response to changes in the impedance of the circuitry.
Although the problems of impedance changes at the output of PA 104 can be avoided by constantly providing a higher than optimum supply voltage to the PA 104, the higher supply voltage leads to a less efficient PA 104. In other words, conventional PA controllers are not able to adjust the power supply for the PA responsive to conditions of output impedance of the PA to maximize the PA efficiency while keeping distortion of the amplified RF signal to an acceptable level.